Mobile devices, such as hand-held computers, RFID readers, and the like, are used in a variety of contexts. Such devices typically include one or more antenna elements to facilitate RF communication.
Modern, high performance radio systems are capable of providing an unusually high receiver dynamic range and high receiver-to-transmitter isolation performance only if they are connected to an antenna system that possesses an extremely small S11 voltage reflection coefficient (e.g., about −30 dB) across an operating band width of, for example, three percent. A UHF mono-static RFID Reader system (one that uses a single antenna for transmitting and receiving) is one example of such a system. A CW Radar system is another example.
An antenna system that meets such stringent performance guidelines represents a considerable deviation from conventional antenna requirements. Traditionally, the Voltage Standing Wave Ratio (VSWR) and the S11 requirements are necessary metrics which assure the transmission efficiency of an antenna. For example, most resonant antennas are manufactured to a specified maximum VSWR of 2:1. This assures a transmission efficiency of at least 89%. Other more stringent antenna designs have a specified maximum VSWR of 1.5:1, which will assure a transmission efficiency of at least 96%. Very few antennas have ever been manufactured to the tighter specification for two reasons: There are very few designs that can maintain that performance over the required frequency range, and such an endeavor impacts the manufacturing yield, and thus the cost.
A relatively recent requirement has emerged for an antenna design with a reflection coefficient S11 of at least −30 dB of S11 (corresponding to a VSWR of 1.065:1) across the operating frequency range, with a worst case performance of approximately −20 dB of S11 (a VSWR of 1.22:1) at the operating band edges.
Prior art antenna designs find it difficult to meet these requirements for a variety of reasons, not the least of which is their relative narrow operating bandwidths. In order to increase bandwidth, a number of techniques known in the art have been attempted. However, these methods are unsatisfactory in that they generate unwanted heat energy, increase the volume of the antenna, decouple the radiators, and/or give rise to additional undesirable results.
Accordingly, there is a need for improved, high-bandwidth antenna systems for use with RFID readers and other mobile devices. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.